Just for the record the majority of Automotive lubricating oil manufacturers are blenders of oils, they obtain their ingredients from the oil majors or large wholesalers. It's only the majors that produce their own lube oils from scratch, like Shell etc, which is not to say they produce the best because of that. THE most important ingredient is the base stock, that it the foundation of the oil, the best quality base stocks are expensive so you get what you pay for. Then come the additive packages, again the best are expensive. These days it's the additives that determine the application of a given oil, the applications can be very specific down to certain models of car in a range.
In the case of Vincents it's the viscosity that's most important and most make the mistake of using too high a grade, the operating temperature of Vin motors is very low in standard form, so as was mentioned above, 20 should be considered the limit in a temperate climate, 5/30 would be a good multigrade choice. Assessing the viscosity as it appears from the can at ambient temperature is a pointless exercise, engine oil is complex chemistry and difficult to analyse without a lab. Shear stability is extremely good with all quality oil these days.
Personally I would not use an expensive synth oil in a Vin, a high quality, general purpose mineral engine oil is more than adequate. My own choice is Morris for classic machines.